1. Field of the Invention
The present invention relates to a device for preventing erroneous synchronization in a digital wireless communication apparatus and, more particularly, to a device for preventing erroneous synchronization in a wireless communication apparatus switchably operating in its normal and power saving modes.
2. Description of the Background Art
As a digital wireless communication system, there is a system, such as the Bluetooth system, made up by a station supervising reference timing, referred to below as a reference or master station, and a dependent or slave station which receives the reference timing to maintain synchronism with the reference station. The slave station operates in its normal mode for performing communication, i.e. in its active state, and in a power saving mode for halting the communication to reduce power consumption, i.e. in its power saving state. The communication link between the reference and slave stations is maintained by acquiring timing sync signals (sync pattern) from each other. If the sync pattern is not acquired within a preset period, the communication link is disconnected. This preset period, which is a parameter dependent upon a system, is set to a default value of 20 seconds in the case of the Bluetooth system.
For example, the reference station transmits a sync pattern defining the reference timing at preset intervals to the slave station. The slave station receives the sync pattern sent from the reference station to maintain synchronism with the reference timing of the reference station. When receiving the sync pattern, the slave station opens a sync window of a preset duration in order to avoid the effect from the ambient noise, etc., as much as possible. The receiving window of a general wireless communication apparatus is standardized so as to allow for timing displacement of received packets on the order of ± several microseconds during the normal mode. This is however not applied to restoration from the power saving mode to the normal mode, but the user may set an arbitrary sync window length. Japanese patent laid-open publication No. 242201/1996 discloses a wireless communication apparatus configured for detecting a word sync character depending on the state of radio waves.
However, if the slave station goes to the power saving mode, the slave station stops acquiring the reference timing of the reference station to set itself the power-down, or sleep, state. In the sleep state, it is not possible anymore to correct the sync timing for assuring synchronism with the reference station. The system clocks of the reference and slave stations usually suffer from drift. If the system clocks of the reference and slave stations are asynchronous relative to each other, the timing of the system clocks of the slave station may be shifted to extend the sync window during the normal mode (on the order of ± several microseconds) from the system clocks of the reference station during this sleep period.
Thus, in reverting from the power saving mode to the normal mode, the slave station opens its receiving window (on the order of ± hundreds of microsecond) broader than the sync window for the normal mode (on the order of ± several microseconds) to receive the sync pattern to maintain the synchronism with the reference station in order to prevent disconnection of the communication link.
However, the above stated wireless communication apparatus suffers from the problem that the communication link may be disconnected by the following causes.
First, during restoration from the power saving mode to the normal mode, the sync window is expanded, with the consequence that the time needed for verifying the sync pattern is prolonged. Consequently, the risk of erroneous synchronization may be higher, e.g. with the ambient noise or with random data transmitted or received between other stations, such that the communication link will be disconnected due to the erroneous synchronization. For example, when operating in the normal mode, the slave station opens its sync window of period of ±10 μs in time with the transmitting timing of the sync pattern included in packets sent from the reference station to receive the sync pattern, as shown in FIG. 8. When reverting from the power saving mode to the normal mode, through the sleep mode, the slave station expands the sync window to ± several hundred microseconds to receive the sync pattern. The slave station then switches itself to the normal mode to restore the sync window of the period of ±10 μs.
However, when the slave station has, as shown in FIG. 9, reverted from the power saving mode to the normal mode and opened the sync window C1 of ± several hundred microseconds, an erroneous detection of, e.g. the ambient noise (asynchronous pattern B1) by the slave station as a sync pattern A1 of the reference station causes the slave station to synchronize its inner timing with the asynchronous pattern B1 and then to switch itself to the normal mode to reset the sync window to the period of ±10 μs. Hence, if the timing of the asynchronous pattern B1 is shifted appreciably from the sync pattern of the reference station, the opening timing of a sync window C2 of the period of ±10 μs, at the time of restoration to the normal mode, is shifted from the timing of the sync pattern A2 sent by the reference station, with the result that the slave station is no longer able to receive the sync pattern of the reference station, thus leading to disconnection of the communication link.
If, during the operation of the slave station in the normal mode, erroneous synchronization has occurred twice or more times on end, such erroneous synchronization gives rise to disconnection of the communication link. During the normal mode, the slave station opens the sync window, in time with the sync pattern periodically sent as packets from the reference station, to receive the sync pattern, as shown in FIG. 10. Since the effect by, e.g. the ambient noise is diminished by setting the sync window to the period of ±10 μs, the probability of the erroneous detection of the sync pattern by e.g. the ambient noise is usually low to a certain extent. However, erroneous detection may still occur, depending on the surrounding states.
For example, as shown in FIG. 11, when a third wireless communication apparatus, not communicating, calls the reference station twice on end, the third wireless communication apparatus transmits the same sync patterns E1 and E2 as the sync patterns D1 and D2 sent from the reference station. Thus, if the sync pattern E1, initially output from the third wireless communication apparatus, is shifted by 8 μs in the positive direction from the sync pattern D1 of the reference station, the timing of the sync pattern E1 falls within the sync window F1 of the slave station, and therefore the slave station detects the sync pattern E1 earlier than the sync pattern D1 from the reference station to synchronize its inner timing with the sync pattern E1.
Consequently, the sync window F2 following the sync window F1 is opened at a timing displaced by 8 μs in the positive direction from the transmitting timing of the sync pattern D2 of the reference station (+18 to −2 μs). Meanwhile, since the retrieval of the sync pattern is usually not carried out twice or more in the same sync window, the sync pattern D1, immediately following the sync pattern E1, of the reference station is not detected.
When the sync window F2 of the slave station is opened, if the second sync pattern E2 is sent from the third wireless communication apparatus to the reference station, and the transmitting timing of the sync pattern E2 is shifted by 11 μs in the positive direction from the sync pattern D2 of the reference station, the sync pattern E2 falls within the sync window F2, so that the slave station erroneously detects the sync pattern E2 as the sync pattern D2 from the reference station, and accordingly synchronizes its inner timing with the sync pattern E2. Consequently, the sync window F3 following the sync window F2 is opened at a timing shifted by 11 μs in the positive direction from the sync pattern D3 of the reference station (+21 to +1 μs), so that the slave station is no longer able to receive the sync pattern from the reference station, with out-of-synchronism then being sustained to lead to disconnection of the communication link.